Increase Wi-Fi signal on Iphone 4?

I'm on Virginmedia

my wi-fi signal gets lost when i go upstairs even thoough my modem is downstairs how do i increase my Wi-Fi signal

i've tried turning my modem off and then on.

1 choice You have to move your modem close to get better signal.

2. You can get range extenders.

3. You can buy a router that avoids interference. Routers like netgear N300 High performance is good they are for multi floors doesn't matter how many walls you have but it won't stop the signal.